A typical tire parameter sensing system for a vehicle includes a vehicle-based unit and a plurality of tire-based units. Each vehicle tire has an associated tire-based unit. Each tire-based unit includes at least one parameter sensor (e.g., pressure, temperature, etc.) and transmission circuitry for transmitting a tire parameter signal indicative of the sensed parameter(s). The vehicle-based unit includes a receiver for receiving tire parameter signals from the tire-based units and a display for providing information to the vehicle operator regarding the sensed tire parameter.
Typically, each tire-based unit includes a long life battery for providing electrical energy to the parameter sensor and transmission circuitry. The batteries typically exhibit an internal resistance, known as an equivalent series resistance. The equivalent series resistance of the batteries varies inversely with temperature.
Vehicles having tire parameter sensing systems are subject to varying conditions. Many vehicles are subjected to extremely cold temperatures during winter months. When the tire-based units are in a low temperature environment, the equivalent series resistance of the batteries can increase to a level that is sufficient to deprive the electronic circuitry of the tire-based unit of a proper operating voltage. For example, the parameter sensor of the tire-based unit may include a microcomputer that operates when supplied with at least two volts of electrical energy. The normal output voltage of the battery is 2.9 to 3.2 volts DC at a nominal operating temperature. When the equivalent series resistance of the battery increases to a particular level as a result of cold temperature, the voltage drop resulting from the equivalent series resistance of the battery may result in the microcomputer receiving less than two volts of electrical energy. As a result, the tire-based unit may become inoperative. It is desirable to control the voltage of the tire-based unit so that decreases in temperature do not result in the tire-based unit becoming inoperative.